The present invention relates to a brake system having slip control. The system includes a pedal-actuated (preferably, auxiliary-force-assisted) braking pressure generator including a master cylinder comprising a longitudinal bore, a primary piston and a secondary piston. The wheel brakes are connected to the braking pressure generator by main brake lines. The system further includes at least one auxiliary-pressure hydraulic pump, as well as wheel sensors and electronic circuits for determining the wheel rotational behavior and for generating electric braking-pressure control signals which, for the purpose of slip control, serve to control electromagnetically actuatable pressure-fluid inlet valves and outlet valves provided in the pressure-fluid lines. Also first and second central control valves are provided whose actuation is effected in opposition to the actuating direction of the master cylinder. Each control valve is provided with a valve-opening mechanism which, in the brake's release position, opens pressure-fluid connections between a pressure fluid supply reservoir and a primary and a secondary pressure chamber and closes these pressure-fluid connections in the braking position.
A slip-controlled brake system of this type is disclosed in the German published patent application P 36 01 914. The hydraulic brake system described therein comprises a master cylinder having a vacuum brake power booster connected upstream thereof and an auxiliary-pressure supply system. Connected to each working chamber of the master cylinder are one or more of the wheel brakes, the auxiliary-pressure supply system and; via a control valve, a pressure-compensating reservoir. The piston in the master cylinder assumes an end position proximate the brake pedal side of the master cylinder when the brake is not applied. In this position, the valve is open and establishes a connection to the reservoir. Upon brake application, the valve closes and remains closed as long as the pressure in the working chamber of the master cylinder (as initiated by the pedal depression) remains less than the auxiliary pressure. Upon commencement of slip control, the auxiliary-pressure supply system is switched on resulting in the delivery of pressure fluid into the working chamber. As a consequence thereof, the piston in the master cylinder is reset to its pedal side end position. The control valve is opened to such an extent and/or for a sufficient period for the forces at the piston to reassume a state of balance. Accordingly, a controlled pressure proportional to the pedal force will prevail in the working chamber after the auxiliary-pressure supply system is activated. By means of switchable multidirectional control valves (namely inlet and oulet valves which are provided in the pressure-fluid conduits from the wheel brakes to the master cylinder and to the reservoir) the braking pressure will be varied when a wheel becomes unstable and the brake slip will be thereby regulated.
In another prior brake system having a vacuum brake power booster, the two hydraulic circuits are separated by an auxiliary-pressure hydraulic pump connected to each individual circuit in order to improve the operational reliability in a slip control action. However, one shortcoming of this known brake system is that the resetting springs of the two pistons in the tandem master cylinder (the master cylinder being connected downstream from the vacuum brake power booster) are not in a position to forcefully open the central control valves during a brake slip control action, during which high hydraulic pressure prevails in the two pressure chambers. Another shortcoming of this known brake system is the considerable overall axial length of the master cylinder that is connected downstream from the vacuum brake power booster, which has adverse effects principally when mounting the braking pressure generator in the engine compartment of an automotive vehicle.